1. Field of the Invention
The present invention is related to a frequency calibration circuit for automatically calibrating frequency and method thereof, and particularly to a frequency calibration circuit and method thereof which utilize a packet identification of a high-speed Universal Serial Bus and/or a packet identification of a full-speed Universal Serial Bus for automatically calibrating frequency.
2. Description of the Prior Art
An oscillator is a significant component found in many electronic products. Periodic waves generated by oscillators may be employed in communication systems, computers or control systems as carriers, timers, counters, or signals with different clocks.
Generally speaking, system vendors widely use less accurate non-crystal oscillators, such as resistor-capacitor (RC) oscillators, instead of crystal oscillators having more accurate oscillation frequencies, such as quartz oscillators, due to lower cost. The prior art uses corrected parameters to program non-volatile memories, such as erasable programmable read only memory (EPROM), electrically-erasable programmable read-only memory (EEPROM), and flash memory, during production of the non-volatile memories, or uses laser-trimming to calibrate the frequency of the RC oscillator. However, the prior art still does not meet a requirement of a high-speed digital system. Therefore, the system vendors are still developing techniques that significantly improve accuracy of oscillation frequencies of the non-crystal oscillators.